1. Field of the Invention
This invention relates generally to switch mode power converters, and more particularly, though not by way of limitation, to soft switching bridge power converters suitable for high power and high voltage DC-DC, AC-DC and DC-AC conversion.
2. Brief Description of the Prior Art
It is generally desirable to operate switching power supplies at the highest frequency that is practical for a particular circuit. Operating at higher frequencies allows inductor and capacitor values in a power supply to be reduced, which reduces physical size and cost and also enables improvements in the transient response of the power supply. Reducing the energy available for delivery to load arcing, such as plasma arcs, is also a desirable goal. High-frequency operation allows the use of smaller output filter capacitors, which store less energy than larger capacitors, and this reduces the energy that can be supplied to plasma arcs. Operating frequencies in switch mode power supplies that utilize hard-switching power converters are limited, however, because the switching losses can become prohibitively high as the operating frequency is increased.
AC-DC and DC-AC bridge power converters typically comprise sets of one or more simple pole circuits. In a hard-switched simple switching pole circuit, for example, a switching device is connected between positive pole and active pole terminals, and a second switching device is connected between the negative pole and active pole terminals. During operation of the pole circuit, the active pole terminal is alternately connected between the positive and negative pole terminals as the switches are turned alternately on and off. A full-bridge converter requires two pole circuits, and a half-bridge converter has only one pole circuit. Bridge converters configured for multiphase operation comprise multiple pole circuits; for example, a three-phase hard-switched bridge converter comprising three simple pole circuits with three active terminals. Depending on how the hard-switched bridge converter it is configured and utilized, power may flow into or out of the active terminals. Switching devices of switching bridge converters are typically realized with active switches (e.g. insulated gate bipolar transistors (IGBT), bipolar transistors, field-effect transistors) or with diodes functioning as passive switches. In hard-switched converter circuits, however, considerable losses may occur when an active switch in a simple pole circuit is turned on while a diode in the other switch of the pole circuit is conducting.
Various schemes have been designed to employ soft-switching inverter poles using auxiliary switches in order to avoid switching losses in power converters. In addition to requiring auxiliary switches, such soft-switching bridge schemes typically include resonant circuits that add additional cost and incur losses due to circulating currents.
Class D amplifier circuits use pole circuits to produce AC or DC voltages or currents that change at rates that are slow compared to the switching frequency. For example, class D audio amplifiers may employ a single hard-switched inverter pole with its active terminal coupled to a load through an LC lowpass filter. Alternatively a DC-DC converter may use two hard-switched inverter pole circuits with active terminals that are connected to coupled filter inductors. The poles circuits are switched in an interleaved manner that reduces ripple in the output current, but the circuit does not provide soft switching.
Like class D amplifiers, opposed-current converters produce AC or DC voltages or currents that change at rates that are slow compared to the switching frequency. These converters may use compound pole circuits that consist of a positive pole circuit and a negative pole circuit. The positive pole terminals are connected together and the negative pole terminals are connected together. An inductor is connected between the active pole terminal of each of the two inverter poles and a node which serves as the active pole terminal for the compound pole circuit. The active switches in the positive and negative pole circuits are both on at the same time, and so considerable current flows in these inductors even when there is no output current, resulting in low efficiencies.